This invention relates to a cutting or reaming tool and more particularly to a tool for reaming a counterbore in a workpiece of the type guided by a pilot member secured in a bore.
In the past, a variety of tools have been designed for cutting or reaming a counterbore in a workpiece. Such tools have been employed in a multitude of applications, including the reaming of a counterbore in the valve block of an internal-combustion engine for the installation of a valve seat ring. Typically, a reaming tool employed in the reaming of a valve seat ring counterbore is utilized with and guided by a pilot member or cutter pilot, and has an annular blade holder on which a plurality of cutter blades are mounted. In use, the cutter pilot, which is elongated and cylindrical, is secured at its bottom end to the underside of the valve block so that its axis is co-extensive with that of the valve bore. The reaming tool is then placed over the free end of the cutter pilot, above the location in which the counterbore is to be cut. A driving tool such as an electrically or pneumatically powered drill is then mounted on the tool and the counterbore is cut by driving the tool rotatably downward along the cutter pilot into the valve block. The cutter blades extend radially outward from the axis of the blade holder a uniform, pre-selected radial distance, and thus ideally, the counterbore is reamed to a pre-selected diameter.
While reaming tools guided by cutter pilots have been generally useful in the past, nevertheless there has remained a problem which limits their usefulness in many applications. The problem is accuracy. In the cutting of a valve seat ring counterbore, for instance, the counterbore being cut or reamed must not exceed a pre-selected diameter, and must be accurately round. If the counterbore exceeds the pre-selected diameter, or is non-circular or out-of-round, the workpiece, i.e., the valve block, may be damaged or destroyed.
In the cutting of valve seat ring counterbores it has been discovered that inaccuracies in the cutting operation result from the gradual wear of the cutter pilot and the blade holder and from deflection or bending of the cutter pilot. That is, as the cutter pilot and the blade holder wear with age, the outer diameter of the cutter pilot is reduced and the inner diameter of the blade holder is enlarged. This difference in diameter means that when a cutter blade mounted on one side of the blade holder encounters material of high hardness, due to the non-homogenity of the material of the valve block, forces acting on the cutter blade shift the blade holder away from the area of hardness and, as a result, cause the other cutter blades to gouge the sidewall of the valve seat ring counterbore being cut. This gouging also occurs when the cutter pilot flexes, and results in a counterbore which may be non-circular and enlarged beyond the desired diameter.
As a further problem the vibration or "chatter" of the cutting tool, which results from gouging during high speed operation, reduces the useful life of the cutters.
Thus, to these and other problems, the present invention is addressed.